Olive oil and mint bashed up in a pestle and mortar with some salt and pepper, added to the potatoes when still hot and drained is all you need. While STILL HOT is key. Dont add mint to the water, its a soft herb and it doesnt really work.

My mum, grand mum and now myself always add fresh mint, stalk and all, to the water in our new potatoes and we always get a wonderful mint flavour. Was your mint fresh? Did you use the whole piece of mint? Mint is like cilantro . . . . much of the flavour is in the stalk.